Since a DVD as an optical disc has digital information recorded thereon in a high density in comparison with a CD as another optical disc, an optical head device for reproducing DVDs has the wavelength of a light source set at 650 nm or 635 nm shorter than 780 nm for CDs or the numerical aperture (NA) of an objective lens set at 0.6 greater than 0.45 for CDs, decreasing the spot size where the light converges on an optical disc surface.
In addition, it has been proposed that the wavelength of the light source be set about 400 nm and the NA be set at not less than 0.6 to obtain a greater recording density in the next generation of optical recording. However, a decrease in the wavelength of the light source or an increase in the NA of the objective lens causes an allowable amount in tilt caused by inclination of an optical disc surface with respect to a direction perpendicular to the optical axis or an allowable amount in thickness variations of an optical disc to decrease.
The reason why these allowable amounts decrease is that the generation of coma aberration for tilted optical discs or the generation of spherical aberration for thickness variations of optical discs degrades the light convergence properties of an optical head device to make reading of signals difficult. There have been proposed several methods for expanding the allowable amounts of optical head devices with respect to tilted optical discs or the thickness variations in case of high density recording.
One of the methods is one wherein the actuator for an objective lens which is usually movable in two axial directions of a tangential direction and a radial direction of an optical disc has an axis for inclination added therein to incline the objective lens in response to a detected tilt angle. However, this addition method creates problems in that the spherical aberration can not be corrected and the structure of the actuator is complicated, for instance.
Another method is one wherein a phase correcting element provided between an objective lens and a light source corrects spherical aberration. This correction method can expand the allowable amount in tilts or thickness variations of optical discs only by incorporation of the element into an optical head device without providing the actuator with significant modifications.
For example, there is JP-A-1020263 directed to the correction method wherein a phase correcting element is utilized to correct the tilt of optical discs. This is a method wherein voltages are applied to divided electrodes that are separately provided on each of a pair of substrates with a birefringent material, such as a liquid crystal, forming a phase correcting element sandwiched therewith, the substantial refractive index of the birefringent material is changed in response to the tilt angle of an optical disc, and the phase (wave front) change of transmitted light caused by the change in the refractive index corrects the coma aberration caused by the tilt of the optical disc.
The conventional phase correcting element requires that the electrodes on the phase correcting element be divided into plural segments, and the respective divided segments have different voltages as different control signals applied thereto in order to change the wave front of outgoing light from the light source to correct wave front aberration. For this reason, many electrodes, wires and external signal sources (power sources) are needed to obtain a desired form of wave front, creating problem in that the structure of the element is complicated, and the use of many external signal sources (power sources) makes the device troublesome. On the other hand, there has been a demand that the numbers of the electrodes, the wires and the external signal sources (power sources) be minimized.
It is difficult to provide a continuous change with a single electrode since the variation in the wave front is even on that electrode. In particular, it has been desired that regions having a great variation in wave front aberration, such as a peripheral portion in spherical aberration, be continuously changed. In addition, the regions between divided electrodes could cause a decrease in transmission rate of light due to, e.g., light scattering since no external signal can be applied to the regions. From these viewpoints, it has been desired that the number of the divided electrodes be minimized, and the number of the regions between the electrodes be minimized.